A swale is a ditch on contour
A keyline is a ditch almost on contour (could be to a
pond in the center or off to the edges)
A drainage ditch like the ones they use to drain swamps are just the same ditches that are off contour.
Sometimes I even call
berms elevated ditches too. But mostly if you have too much water you can get it off the land as soon a possible, or send it all as quickly as possible to a "sacrificial pond area" or you can send from the middle of the land/contour to the end of the land/contour. But overall if your land is already a swamp/lake there is no need to build underwater swales.